Metal oxide particulates are being widely used as a raw material for coating material, matting agent of chemical synthetic fiber, printing ink, cosmetic or milk-white glass, or as a modifier, colorant or pigment for rubber or resin. The surface of a metal oxide particulate is usually covered with a hydroxyl group and assumes to be hydrophilic. Therefore, in the blending of a metal oxide particulate into a resin, a coating material or a cosmetic material which are a hydrophobic material, the metal oxide is surface treated. For example, in the case of titanium oxide, a large number of surface treatments such as surface treatment with a higher fatty acid, surface treatment with an organic silicon compound, a silica-alumina treatment and a surface treatment with various coupling agents, are known and practiced so as to improve dispersibility of the titanium oxide. In the case of aluminum oxide and silicon oxide, attempts are being made to treat the particle surface with a silylating agent or the like.
These methods all are intended to control hydrophobicity or hydrophilicity of the surface or the surface activity by chemically substituting and thereby eliminating the hydroxyl group on the surface of the metal oxide particulate. However, in the case of adding a metal oxide particulate to a fluororesin or a fluorine-based coating material, the metal oxide particulate is preferably more intensified in the hydrophobicity and lipophobicity. For this purpose, the surface must be formed such that fluorine having a surface energy lower than water or oil covers the surface.
In order to solve these problems, for example, JP-A-59-184263 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of adding sodium fluoride to a titanium oxide suspended slurry and stirring the slurry. However, according to this method, when the titanium oxide is a superfine particulate, strong coagulation takes place in the drying step and good dispersibility cannot be obtained even if the coagulate is pulverized. Furthermore, according to this method, the fluorine atom is not bonded directly to the titanium atom, therefore, the titanium oxide surface is not fluorinated in any means.
JP-A-61-215216 discloses a method of using an organic fluorine compound as a hydrophobicity-imparting material in the production of a hydrophobic spherical titanium oxide particulate, however, also in this case, the titanium oxide surface is substantially not fluorinated.
JP-A-3-40919 discloses a method of contacting a fluorocarbon gas with superfine titanium oxide particulate and fluorinating the particle surface at a high temperature of from 200 to 400.degree. C., however, due to the high reaction temperature of 200.degree. C. or more, the thermal efficiency is not necessarily good.
JP-A-6-80901 discloses a pigment (titanium oxide) of which surface is modified with a fluorine gas. According to this method, the pigment can be surface treated at 150.degree. C. or less and dispersibility of the pigment can be improved. However, this surface fluorination method fails in achieving a homogeneous fluorination ratio on the surface of the metal oxide particulate and the dispersibility fluctuates, therefore, in high-level uses, for example, in the case of use as an additive for a water super-repellent coating material or resin, the dispersibility is still in need of improvements.
The object of the present invention is to, by solving the above-described problems encountered in the techniques for producing metal oxide particulates having a substantially fluorinated surface, provide surface-fluorinated metal oxide particulates free of scattering in the physical properties (e.g., dispersibility, water repellency) depending on the particle or particle size at the surface treatment and favored with homogeneously fluorinated surface, and also provide an improved process for manufacturing the metal oxide particulates. More specifically, the object of the present invention is to provide surface-fluorinated metal oxide particulates having higher durability with respect to the physical properties such as water repellency, dispersibility and photostability, and an improved process for manufacturing the metal oxide particulates.
Another object of the present invention is to provide homogeneously surface-fluorinated metal oxide particulates, preferably a homogeneously surface-fluorinated titanium oxide particulate, and an improved thermoplastic resin composition containing the titanium oxide particulate pigment.